A SEM system scans a sample using a focused electron beam to observe the surface state of the sample or analyze the components of the sample using a signal based on or related to secondary electrons or back-scattered electrons emitted from the sample.
An overlay offset does not represent an actual circuit pattern due to a physical distance between the actual circuit pattern and an optical overlay measurement pattern (e.g., an alignment mark) implemented within a scribe line of a wafer. In manufacturing a wafer, process steps are ideally perfectly aligned with one another. A photo scanner performs alignment using an overlay measurement result. When there is misalignment between two process steps, an overlay offset occurs. The overlay offset is compensated for through a compensation process.